At this time we do not have a way to set up a custom background image in a theme for your event registration. The custom theme that we have available allows you to select colors for your event. You can always upload an image as a header image for your event. You can also pass that along as an idea in our feedback area here.
Thank you for posting! Sorry for the confusion. It can take some time to get use to navigating through the new interface. Below I have some screenshots of how to get to your events. I hope this is helpful to you and please let us know if you have any questions.
1. Clink on Campaigns in the upper left-hand side
2. Your Events are now found in one of two places: Campaigns > Activity Type > Events & Registration or Campaigns > Status > Active.